In a 3GPP (3rd Generation Partnership Project) which is a project aiming to standardize a mobile communication system, the introduction of Device to Device (D2D) communication is discussed as a new function after Release 12 (see non patent document 1).
The D2D communication is a communication mode in which a user terminal group including a plurality of adjacent user terminals performs communication without passing through a core network within a frequency band assigned to the mobile communication system.
The D2D communication is assumed to be controlled under the initiative of the base station. Therefore, a user terminal during D2D communication is in a state (connected state) in which the user terminal establishes a connection with a cell in the base station.